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<a href="uniq__id__holder_8hpp.html">Go to the documentation of this file.</a><div class="fragment"><div class="line"><a id="l00001" name="l00001"></a><span class="lineno">    1</span><span class="comment">/* Flow</span></div>
<div class="line"><a id="l00002" name="l00002"></a><span class="lineno">    2</span><span class="comment"> * Copyright 2023 Akamai Technologies, Inc.</span></div>
<div class="line"><a id="l00003" name="l00003"></a><span class="lineno">    3</span><span class="comment"> *</span></div>
<div class="line"><a id="l00004" name="l00004"></a><span class="lineno">    4</span><span class="comment"> * Licensed under the Apache License, Version 2.0 (the</span></div>
<div class="line"><a id="l00005" name="l00005"></a><span class="lineno">    5</span><span class="comment"> * &quot;License&quot;); you may not use this file except in</span></div>
<div class="line"><a id="l00006" name="l00006"></a><span class="lineno">    6</span><span class="comment"> * compliance with the License.  You may obtain a copy</span></div>
<div class="line"><a id="l00007" name="l00007"></a><span class="lineno">    7</span><span class="comment"> * of the License at</span></div>
<div class="line"><a id="l00008" name="l00008"></a><span class="lineno">    8</span><span class="comment"> *</span></div>
<div class="line"><a id="l00009" name="l00009"></a><span class="lineno">    9</span><span class="comment"> *   https://www.apache.org/licenses/LICENSE-2.0</span></div>
<div class="line"><a id="l00010" name="l00010"></a><span class="lineno">   10</span><span class="comment"> *</span></div>
<div class="line"><a id="l00011" name="l00011"></a><span class="lineno">   11</span><span class="comment"> * Unless required by applicable law or agreed to in</span></div>
<div class="line"><a id="l00012" name="l00012"></a><span class="lineno">   12</span><span class="comment"> * writing, software distributed under the License is</span></div>
<div class="line"><a id="l00013" name="l00013"></a><span class="lineno">   13</span><span class="comment"> * distributed on an &quot;AS IS&quot; BASIS, WITHOUT WARRANTIES OR</span></div>
<div class="line"><a id="l00014" name="l00014"></a><span class="lineno">   14</span><span class="comment"> * CONDITIONS OF ANY KIND, either express or implied.</span></div>
<div class="line"><a id="l00015" name="l00015"></a><span class="lineno">   15</span><span class="comment"> * See the License for the specific language governing</span></div>
<div class="line"><a id="l00016" name="l00016"></a><span class="lineno">   16</span><span class="comment"> * permissions and limitations under the License. */</span></div>
<div class="line"><a id="l00017" name="l00017"></a><span class="lineno">   17</span><span class="comment"></span> </div>
<div class="line"><a id="l00018" name="l00018"></a><span class="lineno">   18</span><span class="comment">/// @file</span></div>
<div class="line"><a id="l00019" name="l00019"></a><span class="lineno">   19</span><span class="comment"></span><span class="preprocessor">#pragma once</span></div>
<div class="line"><a id="l00020" name="l00020"></a><span class="lineno">   20</span> </div>
<div class="line"><a id="l00021" name="l00021"></a><span class="lineno">   21</span><span class="preprocessor">#include &quot;<a class="code" href="util__fwd_8hpp.html">flow/util/util_fwd.hpp</a>&quot;</span></div>
<div class="line"><a id="l00022" name="l00022"></a><span class="lineno">   22</span><span class="preprocessor">#include &lt;atomic&gt;</span></div>
<div class="line"><a id="l00023" name="l00023"></a><span class="lineno">   23</span> </div>
<div class="line"><a id="l00024" name="l00024"></a><span class="lineno">   24</span><span class="keyword">namespace </span><a class="code hl_namespace" href="namespaceflow_1_1util.html">flow::util</a></div>
<div class="line"><a id="l00025" name="l00025"></a><span class="lineno">   25</span>{</div>
<div class="line"><a id="l00026" name="l00026"></a><span class="lineno">   26</span><span class="comment"></span> </div>
<div class="line"><a id="l00027" name="l00027"></a><span class="lineno">   27</span><span class="comment">/**</span></div>
<div class="line"><a id="l00028" name="l00028"></a><span class="lineno">   28</span><span class="comment"> * Each object of this class stores (at construction) and returns (on demand) a numeric ID unique from all other</span></div>
<div class="line"><a id="l00029" name="l00029"></a><span class="lineno">   29</span><span class="comment"> * objects of the same class ever constructed, across all time from program start to end.  To be clear, the uniqueness</span></div>
<div class="line"><a id="l00030" name="l00030"></a><span class="lineno">   30</span><span class="comment"> * is not just for all existing objects at a given point in time (for which `this` could simply be used instead) but</span></div>
<div class="line"><a id="l00031" name="l00031"></a><span class="lineno">   31</span><span class="comment"> * across all time.</span></div>
<div class="line"><a id="l00032" name="l00032"></a><span class="lineno">   32</span><span class="comment"> *</span></div>
<div class="line"><a id="l00033" name="l00033"></a><span class="lineno">   33</span><span class="comment"> * ### Copying behavior ###</span></div>
<div class="line"><a id="l00034" name="l00034"></a><span class="lineno">   34</span><span class="comment"> * For obvious reasons, Unique_id_holder cannot have standard copy construction and assignment semantics.</span></div>
<div class="line"><a id="l00035" name="l00035"></a><span class="lineno">   35</span><span class="comment"> * It was contemplated to simply make class noncopyable (forbidding copy construction).</span></div>
<div class="line"><a id="l00036" name="l00036"></a><span class="lineno">   36</span><span class="comment"> * However, then no derived type could use auto-generated copy construction and/or `operator=()`, making the</span></div>
<div class="line"><a id="l00037" name="l00037"></a><span class="lineno">   37</span><span class="comment"> * deriving usage pattern (see below) considerably less concise for such types.</span></div>
<div class="line"><a id="l00038" name="l00038"></a><span class="lineno">   38</span><span class="comment"> *</span></div>
<div class="line"><a id="l00039" name="l00039"></a><span class="lineno">   39</span><span class="comment"> * Therefore, it defines a (somewhat unusual) copy constructor: It is simply equal to the default constructor and simply</span></div>
<div class="line"><a id="l00040" name="l00040"></a><span class="lineno">   40</span><span class="comment"> * generates a unique ID.  The reason: a common pattern (see below) will involve type `C` deriving from</span></div>
<div class="line"><a id="l00041" name="l00041"></a><span class="lineno">   41</span><span class="comment"> * Unique_id_holder.  If `C` happens to use the auto-generated copy constructor implementation, Unique_id_holder&#39;s</span></div>
<div class="line"><a id="l00042" name="l00042"></a><span class="lineno">   42</span><span class="comment"> * copy constructor will be invoked by this implementation.  Since equal (by value) objects are still separate objects,</span></div>
<div class="line"><a id="l00043" name="l00043"></a><span class="lineno">   43</span><span class="comment"> * the correct behavior is for the constructed object to gain a new ID.</span></div>
<div class="line"><a id="l00044" name="l00044"></a><span class="lineno">   44</span><span class="comment"> *</span></div>
<div class="line"><a id="l00045" name="l00045"></a><span class="lineno">   45</span><span class="comment"> * The assignment operator operator=() similarly has unusual behavior: it does nothing, for similar reasons.</span></div>
<div class="line"><a id="l00046" name="l00046"></a><span class="lineno">   46</span><span class="comment"> *</span></div>
<div class="line"><a id="l00047" name="l00047"></a><span class="lineno">   47</span><span class="comment"> * ### Thread safety, performance ###</span></div>
<div class="line"><a id="l00048" name="l00048"></a><span class="lineno">   48</span><span class="comment"> * All operations safe for simultaneous execution on 2+ separate objects or on the same object.</span></div>
<div class="line"><a id="l00049" name="l00049"></a><span class="lineno">   49</span><span class="comment"> * The ID accessor is a totally trivial accessor.  Only construction requires any concurrent-access protection</span></div>
<div class="line"><a id="l00050" name="l00050"></a><span class="lineno">   50</span><span class="comment"> * and internally uses an `atomic` for good efficiency.</span></div>
<div class="line"><a id="l00051" name="l00051"></a><span class="lineno">   51</span><span class="comment"> *</span></div>
<div class="line"><a id="l00052" name="l00052"></a><span class="lineno">   52</span><span class="comment"> * ### Usage patterns ###</span></div>
<div class="line"><a id="l00053" name="l00053"></a><span class="lineno">   53</span><span class="comment"> * This can be used, in particular, to uniquely identify (over all time) objects of any type (across all types).</span></div>
<div class="line"><a id="l00054" name="l00054"></a><span class="lineno">   54</span><span class="comment"> * The most concise usage pattern is probably via public (or possibly private) inheritance.  Another is simply via</span></div>
<div class="line"><a id="l00055" name="l00055"></a><span class="lineno">   55</span><span class="comment"> * composition (storage).</span></div>
<div class="line"><a id="l00056" name="l00056"></a><span class="lineno">   56</span><span class="comment"> *</span></div>
<div class="line"><a id="l00057" name="l00057"></a><span class="lineno">   57</span><span class="comment"> *   ~~~</span></div>
<div class="line"><a id="l00058" name="l00058"></a><span class="lineno">   58</span><span class="comment"> *   void Widget_store::store_in_table(Widget* obj, const Unique_id_holder&amp; obj_id_holder)</span></div>
<div class="line"><a id="l00059" name="l00059"></a><span class="lineno">   59</span><span class="comment"> *   {</span></div>
<div class="line"><a id="l00060" name="l00060"></a><span class="lineno">   60</span><span class="comment"> *     m_unordered_map[obj_id_holder.unique_id()] = obj;</span></div>
<div class="line"><a id="l00061" name="l00061"></a><span class="lineno">   61</span><span class="comment"> *   }</span></div>
<div class="line"><a id="l00062" name="l00062"></a><span class="lineno">   62</span><span class="comment"> *</span></div>
<div class="line"><a id="l00063" name="l00063"></a><span class="lineno">   63</span><span class="comment"> *   // Usage pattern 1: Inheritance.</span></div>
<div class="line"><a id="l00064" name="l00064"></a><span class="lineno">   64</span><span class="comment"> *   class Widget : ..., public Unique_id_holder // Can change to private inheritance if we always register internally.</span></div>
<div class="line"><a id="l00065" name="l00065"></a><span class="lineno">   65</span><span class="comment"> *   {</span></div>
<div class="line"><a id="l00066" name="l00066"></a><span class="lineno">   66</span><span class="comment"> *   private:</span></div>
<div class="line"><a id="l00067" name="l00067"></a><span class="lineno">   67</span><span class="comment"> *     // Example of an object registering itself.</span></div>
<div class="line"><a id="l00068" name="l00068"></a><span class="lineno">   68</span><span class="comment"> *     void some_private_method()</span></div>
<div class="line"><a id="l00069" name="l00069"></a><span class="lineno">   69</span><span class="comment"> *     {</span></div>
<div class="line"><a id="l00070" name="l00070"></a><span class="lineno">   70</span><span class="comment"> *       ...</span></div>
<div class="line"><a id="l00071" name="l00071"></a><span class="lineno">   71</span><span class="comment"> *       s_widget_store.store_in_table(this, *this); // `*this` is itself a Unique_id_holder.</span></div>
<div class="line"><a id="l00072" name="l00072"></a><span class="lineno">   72</span><span class="comment"> *       ...</span></div>
<div class="line"><a id="l00073" name="l00073"></a><span class="lineno">   73</span><span class="comment"> *     }</span></div>
<div class="line"><a id="l00074" name="l00074"></a><span class="lineno">   74</span><span class="comment"> *     static Widget_store s_widget_store;</span></div>
<div class="line"><a id="l00075" name="l00075"></a><span class="lineno">   75</span><span class="comment"> *   };</span></div>
<div class="line"><a id="l00076" name="l00076"></a><span class="lineno">   76</span><span class="comment"> *   ...</span></div>
<div class="line"><a id="l00077" name="l00077"></a><span class="lineno">   77</span><span class="comment"> *     // Example of outside code registering an object.</span></div>
<div class="line"><a id="l00078" name="l00078"></a><span class="lineno">   78</span><span class="comment"> *     Widget_store widget_store;</span></div>
<div class="line"><a id="l00079" name="l00079"></a><span class="lineno">   79</span><span class="comment"> *     Widget widget;</span></div>
<div class="line"><a id="l00080" name="l00080"></a><span class="lineno">   80</span><span class="comment"> *     widget_store::store_in_table(&amp;widget, widget); // `widget` is itself a Unique_id_holder.</span></div>
<div class="line"><a id="l00081" name="l00081"></a><span class="lineno">   81</span><span class="comment"> *   ...</span></div>
<div class="line"><a id="l00082" name="l00082"></a><span class="lineno">   82</span><span class="comment"> *</span></div>
<div class="line"><a id="l00083" name="l00083"></a><span class="lineno">   83</span><span class="comment"> *   // Usage pattern 2: Composition.</span></div>
<div class="line"><a id="l00084" name="l00084"></a><span class="lineno">   84</span><span class="comment"> *   class Widget : ...</span></div>
<div class="line"><a id="l00085" name="l00085"></a><span class="lineno">   85</span><span class="comment"> *   {</span></div>
<div class="line"><a id="l00086" name="l00086"></a><span class="lineno">   86</span><span class="comment"> *   private:</span></div>
<div class="line"><a id="l00087" name="l00087"></a><span class="lineno">   87</span><span class="comment"> *     // Example of an object registering itself.</span></div>
<div class="line"><a id="l00088" name="l00088"></a><span class="lineno">   88</span><span class="comment"> *     void some_private_method()</span></div>
<div class="line"><a id="l00089" name="l00089"></a><span class="lineno">   89</span><span class="comment"> *     {</span></div>
<div class="line"><a id="l00090" name="l00090"></a><span class="lineno">   90</span><span class="comment"> *       ...</span></div>
<div class="line"><a id="l00091" name="l00091"></a><span class="lineno">   91</span><span class="comment"> *       s_widget_store.store_in_table(this, m_uniq_id_holder);</span></div>
<div class="line"><a id="l00092" name="l00092"></a><span class="lineno">   92</span><span class="comment"> *       ...</span></div>
<div class="line"><a id="l00093" name="l00093"></a><span class="lineno">   93</span><span class="comment"> *     }</span></div>
<div class="line"><a id="l00094" name="l00094"></a><span class="lineno">   94</span><span class="comment"> *     Unique_id_holder m_uniq_id_holder;</span></div>
<div class="line"><a id="l00095" name="l00095"></a><span class="lineno">   95</span><span class="comment"> *     static Widget_store s_widget_store;</span></div>
<div class="line"><a id="l00096" name="l00096"></a><span class="lineno">   96</span><span class="comment"> *   };</span></div>
<div class="line"><a id="l00097" name="l00097"></a><span class="lineno">   97</span><span class="comment"> *   ~~~</span></div>
<div class="line"><a id="l00098" name="l00098"></a><span class="lineno">   98</span><span class="comment"> *</span></div>
<div class="line"><a id="l00099" name="l00099"></a><span class="lineno">   99</span><span class="comment"> * Update: A simplified pattern is now possible: If you just want the raw unique number itself and don&#39;t even</span></div>
<div class="line"><a id="l00100" name="l00100"></a><span class="lineno">  100</span><span class="comment"> * want to keep Unique_id_holder objects around or refer to them, call the static create_unique_id() which returns</span></div>
<div class="line"><a id="l00101" name="l00101"></a><span class="lineno">  101</span><span class="comment"> * a new raw `id_t` integer.  The above 2 usage patterns can be rewritten to pass around `id_t` values.</span></div>
<div class="line"><a id="l00102" name="l00102"></a><span class="lineno">  102</span><span class="comment"> * The &quot;Example of outside code registering an object&quot; sub-case is the one exception to that, as for that</span></div>
<div class="line"><a id="l00103" name="l00103"></a><span class="lineno">  103</span><span class="comment"> * either `store_in_table()` must continue to take a Unique_id_holder and not a raw integer, or I suppose</span></div>
<div class="line"><a id="l00104" name="l00104"></a><span class="lineno">  104</span><span class="comment"> * `m_uniq_id_holder` must be made public which some would not like.</span></div>
<div class="line"><a id="l00105" name="l00105"></a><span class="lineno">  105</span><span class="comment"> */</span></div>
<div class="line"><a id="l00106" name="l00106"></a><span class="lineno"><a class="line" href="classflow_1_1util_1_1Unique__id__holder.html">  106</a></span><span class="keyword">class </span><a class="code hl_class" href="classflow_1_1util_1_1Unique__id__holder.html">Unique_id_holder</a></div>
<div class="line"><a id="l00107" name="l00107"></a><span class="lineno">  107</span>{</div>
<div class="line"><a id="l00108" name="l00108"></a><span class="lineno">  108</span><span class="keyword">public</span>:</div>
<div class="line"><a id="l00109" name="l00109"></a><span class="lineno">  109</span>  <span class="comment">// Types.</span></div>
<div class="line"><a id="l00110" name="l00110"></a><span class="lineno">  110</span><span class="comment"></span> </div>
<div class="line"><a id="l00111" name="l00111"></a><span class="lineno">  111</span><span class="comment">  /// Raw integer type to uniquely identify a thing.  64-bit width should make overflow extremely hard to reach.</span></div>
<div class="line"><a id="l00112" name="l00112"></a><span class="lineno"><a class="line" href="classflow_1_1util_1_1Unique__id__holder.html#af9cbe2432658aac398498e0a7e96405f">  112</a></span><span class="comment"></span>  <span class="keyword">using </span><a class="code hl_typedef" href="classflow_1_1util_1_1Unique__id__holder.html#af9cbe2432658aac398498e0a7e96405f">id_t</a> = uint64_t;</div>
<div class="line"><a id="l00113" name="l00113"></a><span class="lineno">  113</span> </div>
<div class="line"><a id="l00114" name="l00114"></a><span class="lineno">  114</span>  <span class="comment">// Constructors/destructor.</span></div>
<div class="line"><a id="l00115" name="l00115"></a><span class="lineno">  115</span><span class="comment"></span> </div>
<div class="line"><a id="l00116" name="l00116"></a><span class="lineno">  116</span><span class="comment">  /// Thread-safely construct an ID whose value is different from any other object of this class, past or future.</span></div>
<div class="line"><a id="l00117" name="l00117"></a><span class="lineno">  117</span><span class="comment"></span>  <span class="keyword">explicit</span> <a class="code hl_function" href="classflow_1_1util_1_1Unique__id__holder.html#a90705293787069f52372638d5450ef52">Unique_id_holder</a>();</div>
<div class="line"><a id="l00118" name="l00118"></a><span class="lineno">  118</span><span class="comment"></span> </div>
<div class="line"><a id="l00119" name="l00119"></a><span class="lineno">  119</span><span class="comment">  /**</span></div>
<div class="line"><a id="l00120" name="l00120"></a><span class="lineno">  120</span><span class="comment">   * This copy constructor is identical in behavior to Unique_id_holder(), the default ctor.  Unique_id_holder doc</span></div>
<div class="line"><a id="l00121" name="l00121"></a><span class="lineno">  121</span><span class="comment">   * header explains this behavior and why copy construction is not disallowed entirely.</span></div>
<div class="line"><a id="l00122" name="l00122"></a><span class="lineno">  122</span><span class="comment">   */</span></div>
<div class="line"><a id="l00123" name="l00123"></a><span class="lineno">  123</span>  <span class="keyword">explicit</span> <a class="code hl_function" href="classflow_1_1util_1_1Unique__id__holder.html#a90705293787069f52372638d5450ef52">Unique_id_holder</a>(<span class="keyword">const</span> <a class="code hl_class" href="classflow_1_1util_1_1Unique__id__holder.html">Unique_id_holder</a>&amp;);</div>
<div class="line"><a id="l00124" name="l00124"></a><span class="lineno">  124</span> </div>
<div class="line"><a id="l00125" name="l00125"></a><span class="lineno">  125</span>  <span class="comment">// Methods.</span></div>
<div class="line"><a id="l00126" name="l00126"></a><span class="lineno">  126</span><span class="comment"></span> </div>
<div class="line"><a id="l00127" name="l00127"></a><span class="lineno">  127</span><span class="comment">  /**</span></div>
<div class="line"><a id="l00128" name="l00128"></a><span class="lineno">  128</span><span class="comment">   * Raw unique ID identifying this object as well as any object of a derived type.</span></div>
<div class="line"><a id="l00129" name="l00129"></a><span class="lineno">  129</span><span class="comment">   *</span></div>
<div class="line"><a id="l00130" name="l00130"></a><span class="lineno">  130</span><span class="comment">   * @return Numeric ID.  Can be used as a key for most classic associative containers without extra code.</span></div>
<div class="line"><a id="l00131" name="l00131"></a><span class="lineno">  131</span><span class="comment">   */</span></div>
<div class="line"><a id="l00132" name="l00132"></a><span class="lineno">  132</span>  <a class="code hl_typedef" href="classflow_1_1util_1_1Unique__id__holder.html#af9cbe2432658aac398498e0a7e96405f">id_t</a> <a class="code hl_function" href="classflow_1_1util_1_1Unique__id__holder.html#a24742b697933e623df30056aebbebfae">unique_id</a>() <span class="keyword">const</span>;</div>
<div class="line"><a id="l00133" name="l00133"></a><span class="lineno">  133</span><span class="comment"></span> </div>
<div class="line"><a id="l00134" name="l00134"></a><span class="lineno">  134</span><span class="comment">  /**</span></div>
<div class="line"><a id="l00135" name="l00135"></a><span class="lineno">  135</span><span class="comment">   * This assignment operator is a `const` no-op.  Unique_id_holder doc</span></div>
<div class="line"><a id="l00136" name="l00136"></a><span class="lineno">  136</span><span class="comment">   * header explains this behavior, and why assignment is not disallowed entirely.</span></div>
<div class="line"><a id="l00137" name="l00137"></a><span class="lineno">  137</span><span class="comment">   *</span></div>
<div class="line"><a id="l00138" name="l00138"></a><span class="lineno">  138</span><span class="comment">   * @return `*this`.</span></div>
<div class="line"><a id="l00139" name="l00139"></a><span class="lineno">  139</span><span class="comment">   */</span></div>
<div class="line"><a id="l00140" name="l00140"></a><span class="lineno">  140</span>  <span class="keyword">const</span> <a class="code hl_class" href="classflow_1_1util_1_1Unique__id__holder.html">Unique_id_holder</a>&amp; <a class="code hl_function" href="classflow_1_1util_1_1Unique__id__holder.html#abb6bab931f868d2a2e933d94f7540b1c">operator=</a>(<span class="keyword">const</span> <a class="code hl_class" href="classflow_1_1util_1_1Unique__id__holder.html">Unique_id_holder</a>&amp;) <span class="keyword">const</span>;</div>
<div class="line"><a id="l00141" name="l00141"></a><span class="lineno">  141</span><span class="comment"></span> </div>
<div class="line"><a id="l00142" name="l00142"></a><span class="lineno">  142</span><span class="comment">  /**</span></div>
<div class="line"><a id="l00143" name="l00143"></a><span class="lineno">  143</span><span class="comment">   * Short-hand for `Unique_id_holder().unique_id()`; useful when all you want is the unique integer itself.</span></div>
<div class="line"><a id="l00144" name="l00144"></a><span class="lineno">  144</span><span class="comment">   *</span></div>
<div class="line"><a id="l00145" name="l00145"></a><span class="lineno">  145</span><span class="comment">   * @return See unique_id().</span></div>
<div class="line"><a id="l00146" name="l00146"></a><span class="lineno">  146</span><span class="comment">   */</span></div>
<div class="line"><a id="l00147" name="l00147"></a><span class="lineno">  147</span>  <span class="keyword">static</span> <a class="code hl_typedef" href="classflow_1_1util_1_1Unique__id__holder.html#af9cbe2432658aac398498e0a7e96405f">id_t</a> <a class="code hl_function" href="classflow_1_1util_1_1Unique__id__holder.html#a50489177a7731618ca130468c3872748">create_unique_id</a>();</div>
<div class="line"><a id="l00148" name="l00148"></a><span class="lineno">  148</span> </div>
<div class="line"><a id="l00149" name="l00149"></a><span class="lineno">  149</span><span class="keyword">private</span>:</div>
<div class="line"><a id="l00150" name="l00150"></a><span class="lineno">  150</span> </div>
<div class="line"><a id="l00151" name="l00151"></a><span class="lineno">  151</span>  <span class="comment">// Data.</span></div>
<div class="line"><a id="l00152" name="l00152"></a><span class="lineno">  152</span><span class="comment"></span> </div>
<div class="line"><a id="l00153" name="l00153"></a><span class="lineno">  153</span><span class="comment">  /// The ID.  Note its `const`ness alone forbids a classic assignment operator.</span></div>
<div class="line"><a id="l00154" name="l00154"></a><span class="lineno"><a class="line" href="classflow_1_1util_1_1Unique__id__holder.html#abfd221f9f9a3498c9f697ede78d99938">  154</a></span><span class="comment"></span>  <span class="keyword">const</span> <a class="code hl_typedef" href="classflow_1_1util_1_1Unique__id__holder.html#af9cbe2432658aac398498e0a7e96405f">id_t</a> <a class="code hl_variable" href="classflow_1_1util_1_1Unique__id__holder.html#abfd221f9f9a3498c9f697ede78d99938">m_id</a>;</div>
<div class="line"><a id="l00155" name="l00155"></a><span class="lineno">  155</span><span class="comment"></span> </div>
<div class="line"><a id="l00156" name="l00156"></a><span class="lineno">  156</span><span class="comment">  /**</span></div>
<div class="line"><a id="l00157" name="l00157"></a><span class="lineno">  157</span><span class="comment">   * #m_id value of the Unique_id_holder object last constructed.</span></div>
<div class="line"><a id="l00158" name="l00158"></a><span class="lineno">  158</span><span class="comment">   *</span></div>
<div class="line"><a id="l00159" name="l00159"></a><span class="lineno">  159</span><span class="comment">   * ### Implementation subtleties ###</span></div>
<div class="line"><a id="l00160" name="l00160"></a><span class="lineno">  160</span><span class="comment">   * Using `atomic` for concise thread safety.</span></div>
<div class="line"><a id="l00161" name="l00161"></a><span class="lineno">  161</span><span class="comment">   *</span></div>
<div class="line"><a id="l00162" name="l00162"></a><span class="lineno">  162</span><span class="comment">   * I don&#39;t specify any memory ordering arguments when using this</span></div>
<div class="line"><a id="l00163" name="l00163"></a><span class="lineno">  163</span><span class="comment">   * because of the simplicity of this implementation (a lone `++`).  Be careful if the code increases in complexity</span></div>
<div class="line"><a id="l00164" name="l00164"></a><span class="lineno">  164</span><span class="comment">   * though; any kind of thread coordination would probably mean one would have to think of such matters and switch</span></div>
<div class="line"><a id="l00165" name="l00165"></a><span class="lineno">  165</span><span class="comment">   * away from just overloaded operator(s) in favor of explicitly-memory-order-specifying `atomic&lt;&gt;` API calls.</span></div>
<div class="line"><a id="l00166" name="l00166"></a><span class="lineno">  166</span><span class="comment">   *</span></div>
<div class="line"><a id="l00167" name="l00167"></a><span class="lineno">  167</span><span class="comment">   * Normally when an `std` implementation is available along with a `boost` one, I choose the latter,</span></div>
<div class="line"><a id="l00168" name="l00168"></a><span class="lineno">  168</span><span class="comment">   * for the likely extra features and for consistency.  I chose to do differently in the case of `atomic`, because,</span></div>
<div class="line"><a id="l00169" name="l00169"></a><span class="lineno">  169</span><span class="comment">   * firstly, this is widely recommended, and, secondly, because this is a low-level feature and &quot;feels&quot; like it</span></div>
<div class="line"><a id="l00170" name="l00170"></a><span class="lineno">  170</span><span class="comment">   * might be particularly well coded for a given architecture&#39;s compiler.  Not that the Boost guys aren&#39;t great.</span></div>
<div class="line"><a id="l00171" name="l00171"></a><span class="lineno">  171</span><span class="comment">   */</span></div>
<div class="line"><a id="l00172" name="l00172"></a><span class="lineno"><a class="line" href="classflow_1_1util_1_1Unique__id__holder.html#ad42e76b8fd63b28a0775167c43edfc58">  172</a></span>  <span class="keyword">static</span> std::atomic&lt;id_t&gt; <a class="code hl_variable" href="classflow_1_1util_1_1Unique__id__holder.html#ad42e76b8fd63b28a0775167c43edfc58">s_last_id</a>;</div>
<div class="line"><a id="l00173" name="l00173"></a><span class="lineno">  173</span>}; <span class="comment">// class Unique_id_holder</span></div>
<div class="line"><a id="l00174" name="l00174"></a><span class="lineno">  174</span> </div>
<div class="line"><a id="l00175" name="l00175"></a><span class="lineno">  175</span>} <span class="comment">// namespace flow::util</span></div>
<div class="ttc" id="aclassflow_1_1util_1_1Unique__id__holder_html"><div class="ttname"><a href="classflow_1_1util_1_1Unique__id__holder.html">flow::util::Unique_id_holder</a></div><div class="ttdoc">Each object of this class stores (at construction) and returns (on demand) a numeric ID unique from a...</div><div class="ttdef"><b>Definition:</b> <a href="uniq__id__holder_8hpp_source.html#l00106">uniq_id_holder.hpp:107</a></div></div>
<div class="ttc" id="aclassflow_1_1util_1_1Unique__id__holder_html_a24742b697933e623df30056aebbebfae"><div class="ttname"><a href="classflow_1_1util_1_1Unique__id__holder.html#a24742b697933e623df30056aebbebfae">flow::util::Unique_id_holder::unique_id</a></div><div class="ttdeci">id_t unique_id() const</div><div class="ttdoc">Raw unique ID identifying this object as well as any object of a derived type.</div><div class="ttdef"><b>Definition:</b> <a href="uniq__id__holder_8cpp_source.html#l00053">uniq_id_holder.cpp:53</a></div></div>
<div class="ttc" id="aclassflow_1_1util_1_1Unique__id__holder_html_a50489177a7731618ca130468c3872748"><div class="ttname"><a href="classflow_1_1util_1_1Unique__id__holder.html#a50489177a7731618ca130468c3872748">flow::util::Unique_id_holder::create_unique_id</a></div><div class="ttdeci">static id_t create_unique_id()</div><div class="ttdoc">Short-hand for Unique_id_holder().unique_id(); useful when all you want is the unique integer itself.</div><div class="ttdef"><b>Definition:</b> <a href="uniq__id__holder_8cpp_source.html#l00058">uniq_id_holder.cpp:58</a></div></div>
<div class="ttc" id="aclassflow_1_1util_1_1Unique__id__holder_html_a90705293787069f52372638d5450ef52"><div class="ttname"><a href="classflow_1_1util_1_1Unique__id__holder.html#a90705293787069f52372638d5450ef52">flow::util::Unique_id_holder::Unique_id_holder</a></div><div class="ttdeci">Unique_id_holder()</div><div class="ttdoc">Thread-safely construct an ID whose value is different from any other object of this class,...</div><div class="ttdef"><b>Definition:</b> <a href="uniq__id__holder_8cpp_source.html#l00031">uniq_id_holder.cpp:31</a></div></div>
<div class="ttc" id="aclassflow_1_1util_1_1Unique__id__holder_html_abb6bab931f868d2a2e933d94f7540b1c"><div class="ttname"><a href="classflow_1_1util_1_1Unique__id__holder.html#abb6bab931f868d2a2e933d94f7540b1c">flow::util::Unique_id_holder::operator=</a></div><div class="ttdeci">const Unique_id_holder &amp; operator=(const Unique_id_holder &amp;) const</div><div class="ttdoc">This assignment operator is a const no-op.</div><div class="ttdef"><b>Definition:</b> <a href="uniq__id__holder_8cpp_source.html#l00047">uniq_id_holder.cpp:47</a></div></div>
<div class="ttc" id="aclassflow_1_1util_1_1Unique__id__holder_html_abfd221f9f9a3498c9f697ede78d99938"><div class="ttname"><a href="classflow_1_1util_1_1Unique__id__holder.html#abfd221f9f9a3498c9f697ede78d99938">flow::util::Unique_id_holder::m_id</a></div><div class="ttdeci">const id_t m_id</div><div class="ttdoc">The ID. Note its constness alone forbids a classic assignment operator.</div><div class="ttdef"><b>Definition:</b> <a href="uniq__id__holder_8hpp_source.html#l00154">uniq_id_holder.hpp:154</a></div></div>
<div class="ttc" id="aclassflow_1_1util_1_1Unique__id__holder_html_ad42e76b8fd63b28a0775167c43edfc58"><div class="ttname"><a href="classflow_1_1util_1_1Unique__id__holder.html#ad42e76b8fd63b28a0775167c43edfc58">flow::util::Unique_id_holder::s_last_id</a></div><div class="ttdeci">static std::atomic&lt; id_t &gt; s_last_id</div><div class="ttdoc">m_id value of the Unique_id_holder object last constructed.</div><div class="ttdef"><b>Definition:</b> <a href="uniq__id__holder_8hpp_source.html#l00172">uniq_id_holder.hpp:172</a></div></div>
<div class="ttc" id="aclassflow_1_1util_1_1Unique__id__holder_html_af9cbe2432658aac398498e0a7e96405f"><div class="ttname"><a href="classflow_1_1util_1_1Unique__id__holder.html#af9cbe2432658aac398498e0a7e96405f">flow::util::Unique_id_holder::id_t</a></div><div class="ttdeci">uint64_t id_t</div><div class="ttdoc">Raw integer type to uniquely identify a thing. 64-bit width should make overflow extremely hard to re...</div><div class="ttdef"><b>Definition:</b> <a href="uniq__id__holder_8hpp_source.html#l00112">uniq_id_holder.hpp:112</a></div></div>
<div class="ttc" id="anamespaceflow_1_1util_html"><div class="ttname"><a href="namespaceflow_1_1util.html">flow::util</a></div><div class="ttdoc">Flow module containing miscellaneous general-use facilities that don't fit into any other Flow module...</div><div class="ttdef"><b>Definition:</b> <a href="basic__blob_8hpp_source.html#l00028">basic_blob.hpp:29</a></div></div>
<div class="ttc" id="autil__fwd_8hpp_html"><div class="ttname"><a href="util__fwd_8hpp.html">util_fwd.hpp</a></div></div>
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